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Conifer species adapt to low-rainfall climates by following one of two divergent pathways

Citation

Brodribb, TJ and McAdam, SAM and Jordan, GJ and Martins, SCV, Conifer species adapt to low-rainfall climates by following one of two divergent pathways, Proceedings of the National Academy of Sciences, 111, (40) pp. 14489-14493. ISSN 0027-8424 (2014) [Refereed Article]

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Abstract

Water stress is one of the primary selective forces in plant
evolution. There are characters often cited as adaptations to water
stress, but links between the function of these traits and
adaptation to drying climates are tenuous. Here we combine
distributional, climatic, and physiological evidence from 42 species
of conifers to show that the evolution of drought resistance
follows two distinct pathways, both involving the coordinated
evolution of tissues regulating water supply (xylem) and water
loss (stomatal pores) in leaves. Only species with very efficient
stomatal closure, and hence low minimum rates of water loss,
inhabit dry habitats, but species diverged in their apparent
mechanism for maintaining closed stomata during drought. An
ancestral mechanism found in Pinaceae and Araucariaceae species
relies on high levels of the hormone abscisic acid (ABA) to close
stomata during water stress. A second mechanism, found in the
majority of Cupressaceae species, uses leaf desiccation rather than
high ABA levels to close stomata during sustained water stress.
Species in the latter group were characterized by xylem tissues
with extreme resistance to embolism but low levels of foliar ABA
after 30 d without water. The combination of lowlevels of ABA under
stress with cavitation-resistant xylem enables these species to prolong
stomatal opening during drought, potentially extending their
photosynthetic activity between rainfall events. Our data demonstrate
a surprising simplicity in the way conifers evolved to cope
with water shortage, indicating a critical interaction between xylem
and stomatal tissues during the process of evolution to dry climates.